This invention relates to an X-ray detector. It relates more particularly to an X-ray detector that can be incorporated in relatively large numbers into high speed tomography apparatus to produce output signals for developing an image representing a two dimensional slice through a patient's body.
In a scanner such as this, a fan-shaped X-ray beam is moved from one source position to another so that the beam repeatedly passes edgewise through a selected slice of a patient's body. The X-rays following different tracks through that slice are absorbed to a greater or lesser degree depending upon the absorbtivities of the particular tissues, bones and fluids which they encounter. Resultantly, the intensities of the X-rays emerging from the body slice contain information relating to the characteristics of the tissues, bones, etc. along the particular tracks followed by the X-rays. An array of detectors positioned opposite the beam source beyond the patient's body measures the intensity of the emergent radiation and the output signals from these detectors correspond to the measured radiation intensities. These signals are correlated and processed by a computer to produce a two dimensional image of that body slice using techniques well known in the art.
If the resultant image is to accurately depict the body slice and to avoid exposing the patient to more than the minimum radiation dosage necessary to collect enough information for the reconstruction, it is essential that the detector array detect a maximum number of -- ideally all of -- the emergent X-rays. In practice, it has been found that due to the extremely wide variation in the densities of the tissues, bones, voids, fluids, etc. in the body, the intensity of the detected radiation can vary over an extremely wide range, e.g. 100,000 to 1 or more. Consequently, the detectors employed in scanners such as this must have a corresponding wide dynamic range.
Also in high speed tomography apparatus with which we are particularly concerned here, the X-ray beam scans at a high rate of speed, e.g. 100 to 1000 X-ray pulses or source positions per second. To obtain a measurement useful for reconstruction, many X-ray photons must strike the detector (e.g. 100,000) during each pulse. Clearly, very many X-ray photons are detected per second in each detector. As a result, it is not feasible to count individual X-ray photons electronically. Instead a current proportional to the X-ray flux is integrated. This current is obtained from the detector which is receiving a visible light flux from the scintillator which is directly proportional to the incident X-ray flux. This current integrating mode of operation places special requirements on the light-sensor's linearity and constancy of response. Further, the light sensor used should not have appreciable offset currents, hysteresis or other time-varying behavior.
A third constraint on the detector is size. Each detector must be quite small, not only because of the large number of detectors required in a typical detector array, e.g. 600 or more, but also because the resolution of the resultant image varies inversely with the size of the detector and directly with the detector packing density.
Tomography scanners conventionally employ X-ray detectors in the form of scintillators which use photomultiplier tubes. These are fairly large devices, e.g. 2.5 cm or more on a side. Therefore in large numbers, they occupy a considerable amount of space. Also the resolution of the image which an array of such detectors can produce is not very high.
In nuclear and cosmic ray research, another type of detector consisting of a luminescing, fluorescing or scintillating crystal optically coupled to a photo-voltiac conversion cell has been employed to detect gamma radiation. However in these applications, invariably the detectors are used in a pulse mode to count individual rays and also to determine their energy. The requirements for a detector used in this way are quite different from those that are important when a detector is used in the integrating mode as required for X-ray tomography, for example.
It has heretofore been proposed to use that general type of crystal/photo-cell detector in diagnostic X-ray applications. Proposals such as this are described, for example, in U.S. Pat. Nos. 2,899,560; 3,415,989; 3,814,938; and 3,932,756. However, the detectors disclosed there are relatively large and expensive and while they might be efficient enough in some diagnostic X-ray applications, they are apparently unsatisfactory for use in connection with tomography since none of them are being used in that application which in recent years has seen burgeoning research and development activity.